1. Field of the Invention
This invention relates to an improvement of a receiving antenna unit for a car navigation device.
3. Related Art
A conventional receiving antenna unit in a car navigation system is as shown in FIGS. 3 and 4. The receiving antenna unit is magnetically held on the surface of an object such as the roof panel X of an automobile when necessary. The receiving antenna unit comprises: a unit casing 2A whose bottom is covered with a bottom cover 1A which is a die casting of aluminum; a circuit board 3A on which a number of circuit elements are mounted; a metal shield cover 11A covering the circuit board 3A; and an antenna body 4A for receiving radio waves from for instance a geostationary satellite. The circuit board 3A, the metal shield cover, and the antenna body 4A are built in the unit casing 2A.
In order to detachably mount the antenna unit on the surface of an object and more specifically, in order to magnetically hold it on the object, a bottom recess 5A is formed in the bottom of the bottom cover 1A, and a back yoke 7A of ferromagnetic material is fixedly set in the bottom recess 5A with a plurality of fixing screws 6A. The back yoke 7A, which is set in the bottom recess 5A, has a plurality of mounting pieces 7a which are extended from its outer periphery, and are suitably bent. Thus, the permanent magnet ring 8A is magnetically held to the surface of the back yoke 7A. That is, the magnetic force of the permanent magnetic ring 8A is utilized to mount the antenna unit on the roof panel X of the automobile.
In the antenna unit thus mounted, the permanent magnet ring 8A is magnetically held on the surface of the back yoke 7A with a gap H between the roof panel X and the permanent magnet ring 8A. That is, the antenna unit is held on the surface of an object such as the roof panel X by the magnetic force of the permanent magnet ring 8A.
However, the antenna unit, being designed as described above, suffers from the following difficulty. When the antenna unit is vibrated greatly for instance during the travelling of the automobile, the permanent magnet ring 8A may be unable to maintain the aforementioned gap H being temporarily separated from the back yoke 7A. In this case, the permanent magnet ring 8A is magnetically stuck onto the surface of the roof panel X. If, under this condition, the antenna unit is removed by hand, then sometimes the permanent magnet ring 8A is left on the roof panel X.